Mongoid 4 update array attribute in a document - arrays

Question

What is the mistake in this code for not be able to update an array within a document?
Model
class Foo
include Mongoid::Document
include Mongoid::Timestamps::Created
field :myarray, type: Array
end
Controller
def add_item
#foo = Foo.find_by(uuid: params[:uuid])
unless #foo.nil?
unless #foo.has_attribute? :myarray
#foo[:myarray] = Array.new
end
#foo[:myarray] << params[:item]
#foo.save
end
end
I am using Rails 4 with MongoId 4 and if I do p #foo before #foo.save I can see #foo correctly changed but for any reason the update is not persisted.

Answer 1

When you say this:
#foo[:myarray] << params[:item]
You're modifying the myarray array in-place so Mongoid probably won't recognize that it has changed. Then when you say #foo.save, Mongoid will look at #foo to see what has changed; but the array reference in #foo[:myarray] won't change so Mongoid will decide that nothing has changed and #foo.save won't do anything.
If you force a new array reference to be created by saying:
#foo[:myarray] += [ params[:item] ] # Or anything else that creates a whole new array
then Mongoid will notice that #foo[:myarray] has changed and #foo.save will send the change down to MongoDB via $set operation on the underlying document.
This looks like the Mongoid version of this ActiveRecord problem with PostgreSQL array columns:
New data not persisting to Rails array column on Postgres
The rule of thumb is "don't edit mutable values in-place, create whole new values instead: copy, edit, replace". That way you don't have to worry about manually managing the "is dirty" flags.